Tannakian properties of unit Frobenius-modules

We show that unit $\mathcal{O}_{F,X}^\Lambda$-modules of Emerton and Kisin provide an analogue of locally constant sheaves in the context of B\"ockle-Pink $\Lambda$-crystals. For example they form a tannakian category if the coefficient algebra $\Lambda$ is a field. Our results hold for a big class of coefficien algebras which includes Drinfeld rings, and for arbitary locally noetherian base schemes.